Samples for blood gas analysis have conventionally been collected in syringes having some means to vent air in the syringe prior to sample collection and to protect the sample from external air after collection. Many prior art devices accomplish venting with a filter across a hollow plunger rod which allows passage of air, but not blood, from the interior of the syringe during collection. U.S. Pat. No. 4,821,738 uses a hydrophobic filter and is exemplary of this art. U.S. Pat. No. 4,424,817 uses filters of paper or styrofoam and, in one embodiment, uses two filters, one hydrophilic and one hydrophobic.
U.S. Pat. No. 5,238,003 includes a perforated or slotted disc having upper and lower faces which enclose a hydrophobic filter. The patent adds additional structure which allows the syringe plunger to be advanced during sample collection and thereby overcomes the problem, imposed by conventional fixed plungers, which may cause insufficient sample collection from a patient having low blood pressure.